Polysilazane coatings

ABSTRACT

A composition and process for manufacturing a camouflaged aircraft component. The process includes providing an aircraft component; applying an uncured coating onto the aircraft component wherein the uncured coating comprises polysilazane resin, at least one pigment, at least one matting agent, and at least one diluent; allowing the diluent to evaporate; curing the coating to provided a cured coating that comprises cured polysilazane, at least one pigment, and at least one matting agent.

Subject to right of the assignee afforded under a Small BusinessInnovation Research (SBIR) program and SBIR Project AF04-127, the U.S.government has a paid-up license in this invention and the right inlimited circumstances to require the patent owner to license others onreasonable terms as provided for by the terms of contract numberFA8650-04-M-5006 which was supported by The United States Air ForceResearch Laboratory at Wright-Patterson Air Force Base.

BACKGROUND OF THE INVENTION

This invention pertains to coatings containing polysilazane, includingcamouflage coatings for aircraft.

In certain types of aircraft, especially certain military aircraft suchas a C-17, the leading edge of titanium (or other metal) slats moveperpendicular to the wing's front spar; hence, the gap between the slatand the pylon widens on extension. The flaps are externally blown by theengine efflux for extra lift. It is believed that many of the coatingproblems on the C-17 are due to the hot blasts of the reverse enginethrusts that thermally damage the current polyurethane camouflagecoating on the titanium slats.

The inventors herein have determined that a need exists for thermallyresistant, low gloss, camouflage coating, especially for the leadingedge of aircraft slats. The inventors have also determined that a needsexists for a coating for aluminum heat shields on the C-130 aircraft orother coated aircraft areas subject to engine efflux.

SUMMARY OF THE INVENTION

This invention provides a solution to one or more of the problems anddisadvantages discussed above.

In one broad respect, this invention is a camouflage composition,comprising: cured polysilazane, at least one pigment, and at least onematting agent.

In another broad respect, this invention is a process for making acamouflage composition, comprising: providing a formulation containingpolysilazane resin, at least one pigment, at least one matting agent,and at least one diluent; allowing at least one diluent to evaporate;and curing the resin to form the polysilazane camouflage composition. Inone embodiment, the curing is accomplished using moisture from air. Inanother embodiment, the curing is accomplished under a nitrogenatmosphere.

In another broad respect, this invention is a camouflaged aircraftcomponent, comprising: an aircraft component having a coating thereon,wherein the coating comprises: cured polysilazane, at least one pigment,and at least one matting agent. In one embodiment, the component is awing slat made of titanium.

In another broad respect, this invention is a process for manufacturinga camouflaged aircraft component, comprising: providing an aircraftcomponent; applying an uncured coating onto the aircraft componentwherein the uncured coating comprises polysilazane resin, at least onepigment, at least one matting agent, and at least one diluent; allowingthe diluent to evaporate; curing the coating to provide a cured coatingthat comprises cured polysilazane, at least one pigment, and at leastone matting agent. In one embodiment, the component is a wing slat madeof titanium.

This invention provides a number of advantages. The coating is a singlecomponent, low viscosity liquid, easily spray applied, and dries in anhour per coating, with the coating completely reacted within 24 hours.In service, the jet blasts during reverse thrusting will have a postcureeffect and generally result in property enhancement by furtherconversion of the polysilazane to silicon nitride or carbide ceramic.The coating to be sprayed is HAP free. In thermogravimetric analysis,some coating exhibited low weight losses such as 1.4% at 800 degreesCentigrade, thus demonstrating excellent high-temperature stability.When thermal cycling at 650 degrees Centigrade was conducted, verylittle color and gloss changes were noted and the resistance to diluentswas excellent.

DETAILED DESCRIPTION OF THE INVENTION

The polysilazane resins used in this invention are generally of formula:R—Si(NSi₂)₃ where R is alkyl of from 1 to 6 carbons, or—(—SiRR₂—NR₂—)_(x)(—SiRR—NR₂—)_(y)—(SiRR—NR₃—)_(z) where R2 is hydrogen;x, y, and z are each independently from 1 to 10; and R₃ is(C_(x)H_(2x))Si(OC_(x)H_(2x+1)) especially —CH₂CH₂CH₂—Si(OCH₂CH₃).Representative examples of such materials include Kion CorporationsML33/C33 and ML33/C12, which include polysilazane oligomers. Molecularweights of these oligomers range from about 700 to about 1,150 g/mol.These materials are low viscosity oligomer resins and the chains containtriethoxy silanes. Upon exposure to atmospheric moisture, the triethoxysilane converts to trisilanol that can condense to form a crosslinkedpolysilazane. Thus, curing can be accomplished in air at roomtemperature, though higher temperatures can be used to acceleratecuring. Such polysilazane resins are described in U.S. Pat. Nos.6,329,487 and 6,534,194, incorporated herein by reference in theirentirety. Thus, in general, the silazanes and polysilazanes may beprepared by ammonolysis, the method comprising the following steps: a)introducing at least one halosilane having at least one Si—H bond intoliquid anhydrous ammonia, the amount of liquid anhydrous ammonia beingat least twice the stoichiometric amount of silicon-halide bonds on thehalosilane, the halosilane reacting with the anhydrous liquid ammonia toform a precursor ammonolysis product and an ammonium halide salt or acidthereof, the ammonium halide salt or acid thereof being solubilized andionized in the anhydrous liquid ammonia thereby providing an acidicenvironment; and b) maintaining the precursor ammonolysis product in theacidic environment for a sufficient time to reduce the number of Si—Hbonds relative to the quantity of Si—H bonds that are incorporated intothe novel silazane and/or polysilazane from the halosilane of step (a).The silazanes or polysilazanes prepared are characterized by repeatingunits of silicon-nitrogen comprising a reduced amount of Si—H bondsrelative to the quantity of Si—H bonds that are incorporated into thesilazane or polysilazane from Si—H bond containing starting compounds.Combinations of different silazane starting materials may be used.

The matting agents used in the practice of this invention typically canalter the surface of a coating in such a way that the light falling onit is scattered in a defined fashion. The matting agent particles standout from the coating, invisible to the human eye. The color of thecoating is not affected to any great extent. Representative examples ofsuch matting agents include inorganic matting agents such assilica-based ACEMATT matting agents from Degussa and silica-basedmatting agents available from Ineos Silicas. The matting agents may varyin size and include materials that are micron sized particles. Forexample, the particles may have an average diameter of from about 0.1 to1000 microns, and in one embodiment from 0.1 to 100 microns.Combinations of matting agents may be used.

The pigments used in the practice of this invention may be of any coloror combination of colors, as well as employed in any pattern orcombination of patterns. The pigments used herein are typicallyinorganic materials. Inorganic pigments can be crystals of metal oxides.This structure is extremely stable, and sets it apart from organicpigments, which are generally composed of carbon, oxygen, and nitrogen.Such pigments include mixed metal oxides that include more than one typeof metal atom along with the oxygen to make the pigment. In general,pigments are produced by the high temperature calcination of high grademetal oxides in a kiln according to given time and temperature profiles.The resulting mixed metal oxide can be milled using a variety ofhigh-energy techniques in order to reduce the particle size. Thepigments used herein are typically stable at high temperatures.Representative examples of such pigments include black and grayinorganic pigments, such as the camouflage inorganic pigment packagesfrom Shepherd Color. The camouflage pigment CM2581 available fromShepherd Color contains a mixture of chromic oxide (2-8%), copperchromite black spinel (20-30%), titanium dioxide (50-70%), zinc ironchromate black spinel (10-15%). Combinations of pigments may be used.

The diluents that are used in the practice of this invention include anydiluents that do not react with the polysilazane resin or othercomponents of the formulations of this invention, and which facilitateadequate formation of sprayable formulations. Such diluents includeaprotic diluents, especially aprotic diluents that are HAP compliant.Representative examples of such diluents include but are not limited tocertain chlorinated hydrocarbons, certain fluorinated hydrocarbons,certain fluorochlorohydrocarbons, including but not limited topara-chlorobenzotrifluoride. Additionally, diluents such as butyl andpentyl propionate have been found especially useful as diluents. A widevariety of aromatic and aliphatic ketones and esters can also be usefulas diluents. Combinations of diluents may be used.

Apply by coating, spraying, brushing, and the like. In one embodiment,the formulation is made for spraying. The resulting coating, when cured,may be of a variety of thicknesses, typically in the range from 0.0025cm to 0.05 cm, though typically less than 0.025 cm.

The amounts of the components of the uncured coating of this inventionare typically 10 to 25 percent by weight of polysilazane resin, 35 to 75percent by weight of pigment and 0.3 to 10% matting agent, and 10 to 50percent by weight of diluent, with all percentages adding up to 100percent. In one embodiment, the amounts of the components of the uncuredcoating of this invention are 10 to 25 percent by weight of polysilazaneresin, 55 to 65 percent by weight of pigment and 1 to 5% matting agent,and 10 to 40 percent by weight of diluent, with all percentages addingup to 100 percent. If the composition includes additional materialsother than polysilazane resin, pigment and matting agent, and diluent,the composition overall percentages may change though the relativepercentages of polysilazane resin, pigment and matting agent, anddiluent will remain as indicated above.

The type of substrates and surfaces on which the coating of thisinvention may be used vary widely. For example, the type of surfacesthat can be treated with the adhesives of this invention includes glass,ceramics, metals, and plastics. A particularly advantageous use of thiscoating is as a coating on aircraft components exposed to hightemperatures including exterior aircraft surfaces such as a wing slatmade of titanium. Similarly, the coating of this invention can be usedon aluminum heat shields on the C-130 aircraft or other coated aircraftareas subject to engine efflux

Other materials may be included in the composition of this invention,including but not limited to flow and leveling agents such as availablefrom BYK, hydrophobic fumed silica such as available from Degussa,alumina fibers such as available from Aldrich, silicon carbide fiberssuch as available from Aldrich, and the like. If used, such materialswill be used in amounts of from 0.01 to about 10 percent of theformulations prior to evaporation of the diluent.

The following examples are provided as being illustrative of theinvention, and are not to be construed as limiting the scope of theinvention or claims hereto. Unless otherwise denoted, all percentagesare by weight.

A number of polysilazane coatings were prepared and tested. For eachrun, the coating was formed from a composition containing (1) a diluent(Oxsol 100, para-chlorobenzotrifluoride), (2) a matting agent (obtainedfrom Gasil or Acematt matting agent from Degussa) and a camouflageinorganic pigment package from Shepherd Color, and (3) liquidpolysilazane resin. The formulations were prepared using a Union ProcessAttritor, which is a grinding mill containing internally agitated media(zirconia oxide). The coatings and the results are shown in thefollowing table. In each run, the polysilazane was Kion Corporations'sML33/C12 polysilazane, which is includes triethoxy silane groups and isa low viscosity oligomer. Upon exposure to atmospheric moisture, thetriethoxy silane group converts to trisilanol that can condense to formcrosslinked polysilazane. The curing was conducted using roomtemperature moisture condensation. After diluent evaporation, the matrixcoatings contain between 50% and 76% pigment concentration.

Thermal Pigment Stability and at 800 C. Temp Thermal matting TGA deltaCycling - Polysilazane agent Wt. hot Adhesion Run Wt. % Wt. % DiluentWt. % Loss plate (psi) Gloss 85 1 16 41 43 2.7 298 2080 0.4 2 20 49 312.3 323 2040 0.3 3 16.9 44.6 38.5 2.6 354.9 1720 0.4 4 18 60 22 1.8337.8 1280 0.6 5 20 40 40 2.8 441 1920 0.2 6 16.9 55.1 28 1.7 399 11200.4 7 16 41 43 2.7 401.3 1840 0.3 8 17.8 48.2 34 2.4 484.6 1440 0.4 9 1662 22 1.5 317.3 640 0.3 10 16 62 22 1.5 317.3 720 0.3 11 20 40 40 2.6401.8 960 1.5 12 16 51.5 32.5 2.1 417.9 720 0.3 13 20 58 22 1.4 378.7880 0.3 14 20 58 22 1.3 398.2 960 0.4

Pull-off adhesion (ASTM Immersion Gloss D4541) Mil-H- ImmersionImmersion Jet Run 60 (psi) 83282 Mil-H-5606 Mil-23699 ReferenceSprayability 1 0.7 880 8 7 9 9 8 2 0.3 1200 8 6 5 6 8 3 0.3 760 6 8 7 87 4 0.9 1100 5 5 7 5 6 5 0.5 920 6 10 9 8 9 6 0.3 480 5 10 10 8 8 7 0.8800 6 5 7 7 8 8 0.4 1040 9 7 6 8 7 9 0.2 1600 8 6 5 7 7 10 0.2 1600 8 66 7 7 11 1.2 1520 6 9 8 6 7 12 0.3 1920 8 8 9 7 6 13 0.3 1520 6 5 8 7 614 0.3 1200 5 6 7 6 7

The 14 runs sprayed well with low-pressure commercial spray gunequipment. For numeric ratings of 1 to 10, 10 is the best value. Thethermal stability tests were performed using a thermal shock chamber,where the test panels were cycled from −50 C for 16 hours to 50 C for 8hours, for 72 hours. The fluid resistance tests were performed accordingto MIL-PRF-85285D, which generally involves immersing samples inhydraulic fluids and lubricating oil for 24 hours at elevatedtemperatures. Four hours after removal, specimens were examined visuallyand at 200× for blistering, softening, or other types of degradation.Highly pigmented coatings appeared virtually unaffected undermagnification, while lower pigment valued samples exhibited slightblistering. The specular gloss measurements were taken with aBYK-Gardner micro-TRI-gloss meter. Mil-PRF-85285C permits that a maximumof 9 for camouflage colors at an 85 degree angle of incidence and amaximum of 5 for a 60 degree angle of incidence. Except for run 6, allformulations were below these limits. The thermogravimetric analyseswere conducted on coatings cured at room temperature and humidity forfive days prior to the analysis. Good correlation with the weight lossfor the polysilazane and the percentage of filler and binder wereobserved.

The formulation of Run 10 was sprayed on titanium then exposed to apropane torch for 1 minute. As son as the torch was removed, the sampleswere then fully immersed in water. The standard Mil-PRF-85285polyurethane coating was completely discolored, but run 10 showed noevidence of being exposed. Extensive darkening of the standard coatingwas evident.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as illustrativeembodiments. Equivalent elements or materials may be substituted forthose illustrated and described herein, and certain features of theinvention may be utilized independently of the use of other features,all as would be apparent to one skilled in the art after having thebenefit of this description of the invention.

1. A camouflage composition, comprising: cured polysilazane, at leastone pigment, and at least one matting agent.
 2. The composition of claim1 wherein the cured polysilazane is made from silazane of formula:R—Si(NSi₂)₃ where R is alkyl of from 1 to 6 carbons, or—(—SiRR₂—NR₂—)_(x)—(—SiRR—NR₂—)_(y)—(SiRR—NR₃—)_(z) where R2 ishydrogen; x, y, and z are each independently from 1 to 10; and R₃ is(C_(x)H_(2x))Si(OC_(x)H_(2x+1)).
 3. The composition of claim 2 where thesilazane is an oligomer of a molecular weight in the range from about700 to about 1,150 g/mol.
 4. The composition of claim 1 wherein the atleast one matting is an inorganic silica-based matting agent.
 5. Thecomposition of claim 1 wherein the matting agent has an average diameterof from about 0.1 to 1000 microns.
 6. The composition of claim 1 whereinthe at least one pigment is a black inorganic pigment, a gray inorganicpigment, or a combination thereof.
 7. The composition of claim 1,wherein the composition is in the form of a coating having a thicknessin the range from 0.0025 cm to 0.05 cm.
 8. A process for making acamouflage composition, comprising: providing a formulation containingpolysilazane resin, at least one pigment, at least one matting agent,and at least one diluent; allowing the at least one diluent toevaporate; and curing the resin to form the polysilazane camouflagecomposition.
 9. The process of claim 8, wherein the curing isaccomplished using moisture from air.
 10. The process of claim 8,wherein the curing is accomplished under a nitrogen atmosphere.
 11. Theprocess of claim 8 wherein the diluent is a chlorinated hydrocarbon, afluorinated hydrocarbon, a fluorochlorohydrocarbon, butyl propionate,pentyl propionate, an aliphatic ketone, an aliphatic ester, or acombination thereof.
 12. The process of claim 8 wherein the amounts ofeach component of the uncured coating is from 10 to 25 percent by weightof polysilazane resin, 35 to 75 percent by weight of pigment and 0.3 to10% matting agent, and 10 to 50 percent by weight of diluent, with allpercentages adding up to 100 percent.
 13. The process of claim 8 whereinthe polysilazane resin is of formula: R—Si(NSi₂)₃ where R is alkyl offrom 1 to 6 carbons, or—(—SiRR₂—NR₂—)_(x)—SiRR—NR₂—)_(y)—(SiRR—NR₃—)_(z) where R2 is hydrogen;x, y, and z are each independently from 1 to 10; and R₃ is(C_(x)H_(2x))Si(OC_(x)H_(2x+1)).
 14. The process of claim 8 where thepolysilazane resin is an oligomer of a molecular weight in the rangefrom about 700 to about 1,150 g/mol.
 15. The process of claim 8 whereinthe at least one matting agent is an inorganic silica-based mattingagent.
 16. The process of claim 8 wherein the matting agent has anaverage diameter of from about 0.1 to 1000 microns.
 17. The process ofclaim 8 wherein the at least one pigment is a black inorganic pigment, agray inorganic pigment, or a combination thereof.
 18. The process ofclaim 8, wherein the polysilazane camouflage composition is in the formof a coating having a thickness in the range from 0.0025 cm to 0.05 cm.19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)